The present invention relates to an external cylindrical grinding unit capable of automatically determining an optimal center axis of a cylindrical substance such as semiconductor monocrystal ingot and aligning the optimal center axis of the cylindrical substance with the rotation axis of the grinding unit.
For example, in a semiconductor wafer manufacturing line, a semiconductor monocrystal ingot which is obtained in a crystal pulling method such as the Czochralski method and the floating zone method has generally a circular cylinder-like shape with two cone-shaped ends. The two cone-shaped ends of the monocrystal ingot are severed from the main body of the monocrystal ingot with the sections approximately normal to the center line of the main body. The cylindrical monocrystal ingot is then sliced into thin wafers in a manner such that the slicing direction is normal to the center line of the ingot. However, due to irregular surfaces of the ingot, the wafers sliced from the ingot would naturally have varying diameters and irregular circumferences. To prevent this, the ingot is ground by an external cylindrical grinding machine into the shape of the right circular cylinder, prior to the slicing operation.
To obtain a right circular cylinder with the maximum diameter from an irregularly contoured cylindrical ingot, it is the primary importance to determine an appropriate optimal center axis, based on which the grinding operation is to be conducted. The ingot is to be rotated about the optimally determined center axis while it is externally ground.
In other words, to attain the best yield of the right cylindrical ingot, the optimum center axis should be determined such that the uniform diameter of the wafers sliced from the cylindrical monocrystal ingot will be the largest possible. To attain this end, methods were proposed which include Japanese Patent Laid Open Publication (Kokai) No. 6017682 (1985), and Japanese Patent Laid Open Publication (Kokai) No. 61-33446 (1986).
According to Japanese Patent Laid Open Publication (Kokai) No. 60-17682, the center axis of a log is determined in the following manner: a log is rotated about a provisional center axis; light beams are applied to the log normally to the axis thereof from a side position; that light beam which is exactly tangent to the surface of the log is detected every time the log is rotated by intervals of a predetermined angle at a plurality of locations taken along the length of the log; a polygon is obtained from the detected tangent beams at each detection location; the maximum right circular cylinder confined by the polygons is obtained; and the center line of the confined maximum right circular cylinder is obtained, which is then taken as the center axis about which the log is rotated and externally ground.
According to Japanese Patent Laid Open Publication (Kokai) No 61-33446, the center axis of a log is determined by first detecting the contours of at least three cross sections taken normally to the provisional center axis at arbitrarily selected locations including the two end faces; finding the radii and the centers of the maximum inscribed circles in the respective cross sections; then obtaining the maximum right circular cylinder confined by the at least three maximum inscribed circles; and finally finding the center axis of the confined maximum right circular cylinder.
However, in the field of semiconductor manufacturing the so-called "centering operation", that is, the operation of determining an optimal center axis of a monocrystal ingot is conducted in a center axis determining unit which is installed separately from the external cylindrical grinding unit. Furthermore, the ingot of which an optimal center axis has been determined is then set into the external cylindrical grinding unit by means of a separate setting unit in such a manner that the determined optimal center axis of the ingot is aligned with the rotation axis of the external cylindrical grinding unit. The installation of these separate units requires a considerable amount of installation space, and the number of operation steps required from the centering to the grinding of the ingot is undesirably large. Also, even though the optimal center axis of the ingot is determined with high precision, it is unavoidable that some error occurs as the ingot is conveyed from the optimal center axis determining unit to be set in the external cylindrical grinding unit by means of the setting unit, so that eventually the axis about which the ingot is rotated for grinding is not identical to the determined optimal center axis.